An unusual coalition of environmentalists, watermen and former officials yesterday filed suit against the U.S. Environmental Protection Agency, asking a judge to overhaul the floundering government campaign to clean up the Chesapeake Bay.

The group, led by the nonprofit Chesapeake Bay Foundation, is, in effect, suing the EPA for breach of contract. EPA leaders have signed two federal-state agreements that promised a cleaner Chesapeake. But so far, despite 25 years and nearly $6 billion in spending, they have failed to deliver it.

The lawsuit, filed in U.S. District Court in Washington, asks the court -- without many specifics -- to order the EPA to clamp down harder on polluters.

"If the bay is to be saved, EPA must . . . enforce the rule of law. Is that too much to ask of our government?" said William C. Baker, the bay foundation's president. He said the group's action could be "the most significant lawsuit ever filed in the history of the Chesapeake Bay restoration."

But legal experts said yesterday that it might be difficult for a court to turn around the complex Chesapeake cleanup. And even Baker said the suit was designed to serve as a political marker, an attempt to force the Obama administration to treat the Chesapeake as a priority.

"This lawsuit will put the issue of Chesapeake Bay and clean water squarely on the desk of the new EPA administrator," Baker said.

Benjamin H. Grumbles, an EPA assistant administrator, said yesterday that the foundation's lawsuit asked for some things that the agency could not deliver on its own.

The suit asks, for instance, for new limits on pollution that comes downstream from city storm-sewer systems. Grumbles said that would require cooperation from state and local governments.

"I fear that the lawsuit itself could impede the cleanup. I am concerned that it will divert energy and attention away from the watershed and into the courtroom," Grumbles said.

The suit represents a shift in tactics for the bay foundation, the Chesapeake's best-known advocacy group, which has often operated through compromise and lobbying rather than legal confrontation.

Other plaintiffs include the Maryland and Virginia state watermen's associations, a group of recreational anglers, and former government officials from Virginia, Maryland and the District. These groups have been on opposite sides of previous Chesapeake fights, often over crabbing and fishing limits. Environmentalists supported a proposal by state regulators last year to cut back significantly the number of blue crabs that could be caught in the Chesapeake; watermen said the limits could have a devastating impact on parts of their industry.

Bernie Fowler, a former Maryland state senator, said at a news conference that the group was frustrated by the slow pace of the cleanup, which has failed to improve the bay's most vexing problem: pollution-driven "dead zones" where fish and crabs can't breathe.

Fowler referred to recent stories in The Washington Post in which former leaders of the Chesapeake cleanup conceded that they had sought for years to conceal the scale of the effort's failure.

"My, what a shame!" Fowler said. "Today is a day of reality . . . when pretending we're doing something is over."

The legal complaint hinges on a series of Chesapeake Bay Agreements, in which the EPA and state governments promised to reduce pollution flowing downstream from farms, suburbs and sewage plants. By not living up to these agreements, the suit says, EPA Administrator Stephen L. Johnson has acted in an "arbitrary and capricious" manner.

The suit asks for cuts in pollution from sewage plants, power plants and storm sewers and for better programs to fund cleanup measures on farms. Baker said the ultimate aim is a far cleaner bay within five years.

That's a tall order, said University of Michigan law professor David M. Uhlmann. Environmental suits work best, he said, when they have specific demands: to stop a harmful practice or to follow a well-defined cleanup plan.

"What they're asking for is something much broader," Uhlmann said. "I'm not optimistic about the ability of a lawsuit like this, by itself, to lead to significant change. . . . It's not clear to me what a federal district judge could do" to solve a problem that big.